Modems like Digital Subscriber Line (DSL) modems are normally connected to a device for providing access to a data communication network. Such a device is typically a Digital Subscriber Line Access Multiplexer (DSLAM) and the communication network may be the Internet. One particular type of standard that may be of interest is Very-high-speed Digital Subscriber Line 2 (VDSL2).
When being connected in this way the modems are connected to the device via separate communication lines, typically conductor pairs made of copper. These lines are furthermore often bundled together in a cable. This means that the communication lines are often placed very close to each other. There is in this regard a problem in that a communication line may be subject to crosstalk from one or more neighbouring communication lines. This limits the communication capability in that the rate at which data is transmitted is limited.
There has in recent years evolved techniques for reducing the influence of crosstalk, for instance in relation to VDSL2. VDSL2 has been standardized by the Telecommunication Standardization Sector of the International Telecommunications Union (ITU-T) in recommendation G.993.2.
ITU-T has issued a further recommendation G.993.5, specifying vectoring for VDSL2. Vectoring is a technique for Far-end crosstalk (FEXT) cancellation where the transmission and/or reception on communication lines where VDSL2 is used are jointly processed at the DSLAM side. In the downstream direction pre-coding is used, which pre-distorts the transmitted signals in such a way so that the crosstalk into other lines is cancelled as the signal propagates along the cable.
In the upstream direction the received signals are post-processed to cancel the FEXT. The VDSL2 FEXT crosstalk is the stationary noise that most severely limits the performance of VDSL2 systems. The vectoring recommendation provides a way to estimate the FEXT channel in both downstream and upstream and utilize the estimated crosstalk channel to cancel the crosstalk. Vectoring enables the offering of 100 Mbps from the last hundreds meters to users over DSL lines.
This technique thus provides a significant improvement for DSL modems.
One important application for vectoring technology will be FTTCab (Fiber-To-The-Cabinet). Usually, a few hundreds of lines are connected from a cabinet to the modems of the users. In the cabinet, the lines are connected to a DSLAM unit, which may comprise several DSLAMs. The hundreds of lines going out from the cabinet are normally separated in a few cables of up to 100 lines in each cable. Crosstalk between the cables is usually negligible. However, a very large vectoring system is needed if the lines from each cable can not be separated and connected to separate smaller vectoring systems. The typical number of lines of one vectoring system at the cabinet that operators request is 192 or 384, which are the maximum number of lines served by one cabinet in most cases. Such a vectoring system may comprise of several DSLAMs coordinated by a vectoring engine.
The coefficients in downstream pre-coder and upstream crosstalk canceller are usually updated by an adaptive algorithm, e.g. least mean square (LMS), assisted by channel estimation. For a large vectoring system, channel estimation takes a very long time. For example, for 192 lines, it can take about 16 seconds for one round of channel estimation, without considering feedback time and processing time. This would result in long training time during vectoring initialization and long tracking time in showtime. This may be undesirable in some scenarios and unacceptable for some users.
There is therefore a need for improvement in this regard.